Broke is defined as partly or completely manufactured paper or paper board that is discarded from any point in the manufacturing or finishing process. The term also applies to the furnish made by repulping these materials. Wet broke is taken from the forming and pressing sections, while dry broke emanates from the dryers, calendars, reel, winder and finishing operations in a paper mill.
A broke handling and repulping system is an essential feature on any paper machine. Broke may also include intramill paper and paper board, purchased pulp, waste papers, as well as other cellulosic materials that may be suitable for use in the preparation of pulp for use in the manufacture of paper.
Pulp may be defined as wood fibers capable of being put into a slurry or suspended and then deposited on a screen for filter to form a sheet of paper. The methods employed to accomplish the pulping step typically involve chemical and/or mechanical treatment to reduce the feedstock to individual pulp fibers. In broke pulping, such treatment is directed to paper and paper board derived from trimmings, damaged paper, purchased pulps or paper stocks, and the like. Broke is extremely valuable and is returned to the paper production process by mixing it with the pulp furnish.
Broke is typically repulped in a batch or continuous process in a piece of machinery which disintegrates the paper by combining the broke with water and an alkaline material and subjecting the mixture to suitable agitation to reduce the paper into its individual cellulosic fibers. As a result of using the alkaline material, to break down the pulp, the pulp slurry has a relatively high pH which must be neutralized before the pulp is reintroduced into the paper process.
A typical broke pulping system is shown in FIG. 1. The typical pulper comprises a vessel 10, impeller 12, a recirculation line 14, a broke chute 18, a dump line 20, a water inlet 22, an alkaline material inlet 24, and a sulfuric acid inlet 26. In the traditional system, bails of broke are weighed and dropped into vessel 10 via broke chute 18 along with a suitable quantity of white water and an alkaline material, such as a sodium hydroxide solution. Impeller 12, is then initiated, agitating the mixture of broke, water, and sodium hydroxide solution, at a sufficient speed to provide a shearing force sufficient to break down the pulp. The impeller 12, is driven in a flow pattern such that the a portion of the pulp slurry is drawn through the impeller and into outlet line 16 to recirculation line 14 whereby said portion of the pulp slurry is recirculated back into vessel 10. Vessel 10 may be heated, such as by the addition of steam, through line 28, or alternative heating means known to those of skill in the art. After pulping is completed, the pulp slurry is discharged into a dump chest via dump line 20. Prior to removing the pulp from the vessel 10, the pulp is neutralized by adding an acid, such as sulfuric acid, through the recirculation line 14, dump line 20, or directly metered into the vessel 10.
Sulfuric acid is generally preferred for use in neutralization by the pulp and paper industry, but presents material handling, corrosion, safety, and environmental problems. Due to such manufacturing, safety and environmental concerns, alternative materials and processes are being sought to replace the use of acids such as sulfuric acid in the neutralization process in broke pulping.
Although it is well known that carbon dioxide may be used as an acidifying agent in aqueous systems, it has not been used in the neutralization process in broke pulping due to its expected low efficiency in such a high temperature environment and its cost relative to sulfuric acid. Until the present invention, carbon dioxide was also not considered as a viable alternative for acids such as sulfuric acid in the typical neutralization process in broke pulping due the relatively poor mixing and limited contact time of the acidifying agent with the pulp slurry.